1. Field of the Invention
This invention relates to a switching circuit for a reluctance machine.
2. Description of Related Art
FIGS. 1(a) and 1(b) illustrate a typical 3-phase switched reluctance (SR) machine and a common electronic switching circuit which may be used to control the machine. The machine includes a stators defining salient stator poles 1, 1', 2, 2', 3, 3' on which are wound phase windings w, of which only one is shown in association with a set of poles 2, 2', and a rotor r with salient poles 4, 4' and 5, 5'. It should be noted that this doubly salient machine is quite different in its characteristics and performance from singly salient reluctance machines. The latter are generally referred to as synchronous reluctance machines and operate on sinusoidal or quasi-sinusoidal voltage and current waveforms.
Electronic switching circuits are arranged to supply unidirectional current to the phase windings w. In the switching circuit, each phase winding in the machine of FIG. 1(a) is associated with a circuit leg comprising a pair of electronic switches in series with each winding w across a dc supply. A description of switched reluctance machines, and their design and control can be found in the paper `The Characteristics, Design and Applications of Switched Reluctance Motors and Drives` by Dr. J. M. Stephenson et al., incorporated herein by reference, presented at PCIM '93 Conference and Exhibition at Nurnberg, Germany between Jun. 21st and 24th, 1993.
The skilled person will be aware that an SR motor requires a bipolar excitation voltage applied to the phase winding so as to force the flux in the phase winding up or down as and when required, according to the timing of the control strategy used. However, because the flux in a given phase is generally unipolar, the current need not reverse during a phase period. It is commonly accepted that SR machines are conventionally run with unipolar currents.
The phase currents in, for example, a 3-phase SR machine are generally spaced by 120.degree. in respect of their fundamental frequency component, but they do not always sum to zero due to the non-sinusoidal shape caused by their harmonic content. This means that a 3-phase switching circuit for an SR machine cannot necessarily use a conventional star(wye)- or delta-connected inverter by which to derive dc power from an ac source.
A well-known circuit suitable for switched reluctance machines, which can be operated in several ways, is shown in FIG. 1(b). In the first method of operating the circuit, both switches t of a leg are switched on and off together so that at switch-off the current transfers from the switches to flow through the diodes d. In the second method, only one of the two switches t is opened so that the current circulates, or "freewheels", due to the stored magnetic energy associated with the winding through one switch and one diode. Both switches are turned off at the end of a phase conduction period.
At low speed, the current in the phase winding is typically controlled by chopping, in which case the machine is said to be `current fed`.
FIGS. 2(a) and (b) illustrate typical chopped motoring and generating phase winding current waveforms, respectively, without freewheeling. The current is illustrated in relation to angle .theta. of rotation of the rotor with respect to the stator.
At higher speeds, the time required for the growth and decay of flux is significant in relation to the phase period (defined as the time corresponding to one cycle of phase inductance variation). The time rate of change of flux linkage is determined by the voltage applied to the winding, and the rate of change with respect to angle therefore falls as the speed rises. At these higher speeds there is, therefore, only a single pulse of current in the switches and diodes in each phase period. Corresponding phase winding currents are illustrated in FIGS. 3(a) and (b), respectively, for motoring and generating operation. Operation in this manner is called the "single-pulse" mode of operation in which the machine is said to be `voltage fed`.
It should be noted that the `conduction angle` .theta. in FIGS. 3(a) and (b) is the angle over which the switches are closed: .theta..sub.on is the `switch-on angle` and .theta..sub.off is the `switch-off angle`. Furthermore, the effect of the phase winding resistance has been assumed to be negligible in constructing the waveforms of FIGS. 3(a) and (b). The flux linkage waveform .psi. of the phase winding is illustrated by the broken lines. Following closing of the switches t in FIG. 1(b) associated with a phase winding, the flux linkage grows linearly. When the switches are opened, the flux linkage falls linearly, the current flowing through the diodes d imposing a voltage of -V.sub.s on the windings.
In order to maintain the torque developed by the machine as the speed rises under single-pulse control, it is necessary to maintain the flux amplitude. This is commonly achieved by increasing the conduction angle with speed.
Control of this single-pulse mode of operation with full torque capability at higher speeds is exercised by variation of the angle of a rotor pole relative to a particular stator pole at which the switches are closed (the switch-on angle) and the angle over which the switches remain closed (the previously mentioned conduction angle).
The circuit of FIG. 1(b) is well-suited to controlling the current in the inherently inductive windings of an SR machine. Turning both switches on applies the full supply voltage to the winding of a particular leg, forcing the flux (and, therefore, the current) to the required value at the maximum possible rate. Opening both switches then brings the diodes into conduction and forces the flux (and the current) down quickly. Opening only one switch provides a freewheel path for the winding current, with only a small negative winding voltage--flux then falls at only a relatively low rate. By incorporating these three modes into a suitable control strategy, the winding flux and/or current can be controlled effectively and with relatively low switching losses. Stored energy in the winding (as at the end of a phase period when the switches are turned-off) is returned to the supply via the diodes.
The circuit of FIG. 1(b) is also fault-tolerant. Because the winding is placed between the two switches, there is no possibility of a direct short circuit across the dc supply.
There are, however, drawbacks to the circuit of FIG. 1(b). Firstly, it requires four power terminals for each leg (or phase). Secondly, each phase requires a minimum of two separate cables to connect it to the motor. A third problem is the lack of medium- and high-power preassembled semiconductor modules with the required circuit configuration.
The second of the above drawbacks (the need for at least two cables per phase) can be overcome by using the so-called H circuit shown in FIG. 4. In this circuit the switched reluctance machine windings are connected to a common center point between the two smoothing capacitors C. Half the supply voltage is then available for inducing flux growth and half is available for forcing the flux down. The inherent operation of the basic H circuit is restricted to voltage (single-pulse) control of an SR machine having an even number of phases. The even number of phases is required because some of the energy drawn from, say, the top smoothing capacitor to the winding of phase A is returned to the lower capacitor at switch off. Unless this energy transfer is matched, by an equal one in the other direction from, say, the winding of phase B, the midpoint voltage on the capacitors will drift, eventually reaching the top rail voltage. Given an even number of phases, the circuit still will not work if the windings are energized in a current controlled (chopping) mode because there will inevitably be slight mismatches between the phase winding currents. In the voltage controlled (single pulse) mode, the circuit is self-stabilizing because any small drift of the capacitor midpoint potential results in an increased current drain from the capacitor having the higher voltage. However, with current control, no such stabilizing mechanism exists during repeated chopping cycles, and external circuitry, such as that described in EP-A-0074752, incorporated herein by reference, must be used to regulate the midpoint potential.